(a) Technical Field
The present invention relates to a process for removing acetylenes from olefins by using an ionic liquid-based solution, and particularly to a process of efficiently removing a small amount of acetylenes contained in olefins by using an ionic liquid-based solution where copper halide (CuX, X=halogen atom) is completely dissolved.
(b) Background Art
Olefins are obtained mainly by cracking naphtha or natural gas. However, it requires a rather complex purification process to obtain pure olefins because acetylenic compounds with similar boiling points are produced along with paraffinic hydrocarbons during the cracking process. The acetylenic compounds serve as poison in the process of producing polyolefins and can also lower the product quality. The acetylenic compounds are often converted into solid during the polymerization process, thus blocking the fluid stream and even leading to explosion. For this reason, it is necessary to remove even a small amount of acetylenic compound contained in olefins.
The commercial processes employ catalytic hydrogenation of acetylenes to obtain olefins therefrom. However, olefins produced from the hydrogenation of acetylenes as well as feed olefin can undergo further hydrogenation reaction to produce paraffins, thus resulting in the loss of olefins. Therefore, the removal of acetylenic compounds by using hydrogenation requires a catalyst that can selectively hydrogenate acetylenes. Although catalyst prepared by impregnating palladium impregnated on α-alumina is widely used, additional step for recycling the catalyst is required as disclosed in U.S. Pat. Nos. 3,755,448 and 3,792,981 because a relatively high hydrogenation activity results in the overproduction of paraffins and serious catalyst poisoning is caused by carbon deposit.
Examples of the conventional process of removing acetylenic compounds include the low temperature distillation, the liquid absorption, the solid absorption and the membrane separation in addition to the hydrogenation reaction. The low temperature distillation and the liquid absorption are widely used for separating unsaturated compound such as monoxide and olefins from gas mixture. However, he low temperature distillation requires high cost for equipments and high cost for their operation. The conventional liquid absorption method is also disadvantageous in an economic aspect due to the significant solvent loss.
Although U.S. Pat. Nos. 4,019,879 and 4,034,065 disclose a process for removing unsaturated compound such as carbon monoxide by adsorbing them with molecular sieves, adsorption capacity is limited and this process requires high vacuum during desorption step. U.S. Pat. No. 4,717,398 discloses a process for removing an unsaturated compound by using a faujasite-type zeolite substituted with Cu+ and a pressure swing adsorption method. However, acetylenic compound can react with Cu+ or Ag+, thus producing unstable and explosive copper-acetylide or silver-acetylide.
German patent No. 2,059,794 discloses a process for removing unsaturated compound containing acetylenes by using a liquid-phase absorbent comprising monovalent copper compound and alkanol amine (e.g., monoethanol amine) as main ingredients. However, this process requires an additional purification process because final product is often contaminated by an alkanol amine. Ind. Eng. Chem. Res. 2571 (1998) also discloses a process of separating unsaturated compounds from paraffin mixtures by using a solution comprising silver or copper salt, which reversibly reacts with olefin and acetylene. However, the recycle of the solution requires a complicated and difficult process.
U.S. Pat. No. 3,758,603 discloses a process of separating unsaturated compounds and compounds by using a liquid membrane prepared by impregnating silver salt in a microporous membrane. However, according to this liquid-membrane method, it is difficult to maintain the separation efficiency for a long period of time because a facilitated transport carrier salt can be lost by the introduction of gas and solvent is easily evaporated. U.S. Pat. No. 4,318,714 discloses an ion exchange resin membrane having cations substituted with silver ion (Ag+) for preventing the loss of silver ion. However, this membrane requires the supply of a certain level of water during the separation to facilitate the transportation of target compounds. This membrane is also disadvantageous in that the water contained in the permeated olefins should be removed after the separation.